The therapeutic efficacy of tryptophan and tyrosine in the treatment of depression has been inconsistent. According to this review article, studies have shown that two subgroups of depressed patients can be delineated. The first subgroup (Group A) has low urinary levels of the norepinephrine metabolite MHPG (3-methoxy-4-hydroxyphenethylene glycol). This group fails to respond to amitriptyline, but shows a favorable response to desipramine or imipramine (which tend to raise norepinephrine levels rather than serotonin). They also exhibit mood elevation after receiving dextroamphetamine. The second group (Group B) has normal or high urinary MHPG levels, fails to respond to imipramine, but responds to amitriptyline (which tends to raise brain levels of serotonin, rather than dopamine or norepinephrine). These patients experience no mood elevation after dextroamphetamine. Group A would be expected to respond to L-tyrosine (a norepinephrine precursor) and group B should respond to L-tryptophan (a serotonin precursor). The biochemical separation of these subgroups may increase the therapeutic predictability of both L-tryptophan and L-tyrosine.

Comment: In my experience, L-tryptophan is effective against depression more often than is L-tyrosine. A careful history, however, may help to identify those cases in which L-tyrosine would be the appropriate amino acid to prescribe. I typically ask depressed patients what medications they have received and which ones have been helpful; I also ask whether they have taken amphetamines and how they have responded to them. It appears, as Buist has suggested, that the information obtained from those questions can help predict who will respond to which amino acid.

Buist RA. The therapeutic predictability of tryptophan and tyrosine in the treatment of depression. Int Clin Nutr Rev 1983;3(2):1-3.